1. Field Of The Invention
The invention relates to an apparatus for producing laser radiation.
2. Prior Art
Laser arrangements, wherein the laser mix in the resonator is excited transversely with respect to the optical axis at atmospheric pressure, are known as TEA-lasers (Transversely Excited Atmospheric pressure lasers) and are nowadays the most important developments in the laser art. Examples of that type of laser are the CO.sub.2 TEA-lasers in the infra-red spectral range and, for the visible and near ultra-violet spectral range, respectively, the N.sub.2 -lasers and Excimer-lasers. A common factor in all these lasers is a relatively simple mode of operation and a high degree of efficiency. TEA-lasers having a high pulse frequency are of great interest in regard to technical-scientific uses, for example, in laser chemistry, material processing and lidar experiments. On the one hand, they have a high level of mean output which is comparable to the longitudinally excited continuous CO.sub.2 -laser and, on the other hand, they have a high pulse peak output.
In the TEA-laser, the laser gas mixture is excited by a homogeneous electrical discharge at from about 30 to 50 kV voltage between two oppositely disposed electrodes which are elongate in the direction of the optical axis. In this arrangement, it is important for the discharge to occur uniformly between the electrode surfaces and not to contract to a spark as the laser gas is then partially overheated and moreover not excited at all. Necessary conditions to provide for uniform discharge at atmospheric pressure are the use of electrode pairs with a Rogowski or Chang profile, a lowinductance construction in respect to the discharge circuit, comprising high-tension capacitors, high-tension switches and supply leads to the laser electrodes, and pre-ionization of the laser gas.
Pre-ionization firstly produces free charge carriers which are distributed homogeneously over the discharge space. The main discharge which then begins shortly thereafter can then spread quickly and uniformly over the entire volume between the electrodes. The first TEA-laser by R. Dumanchin and the later Lamberton-Pearson laser and almost all TEA-lasers which are in use today have the common feature that ultra-violet radiation or electron beams are used for producing the pre-ionization [for example, Appl. Phys. Letters, 19, 506, (1971)]. In that case, the UV-radiation is produced by a preliminary discharge by a trigger wire to the anode by means of auxiliary capacitors. The introduction of an auxiliary gas comprising tri-n-propylamine which is mixed with the laser gas increased the effectiveness of this arrangement by virtue of its low ionization potential. Other UV-pre-ionization systems comprise spark trains which are arranged on a carrier in series or over entire surfaces, either beside the electrodes or directly in an electrode. A disadvantage with all these spark train systems is that they are subject to wear and generally have to be changed after a few thousand discharges. In addition, the pre-ionization systems require additional capacitors, resistors and pre-trigger means such as time delay circuits, a second high-tension switch, etc. In addition, most of the pre-ionization systems impede the uniform flow of laser gas between the laser electrodes. This is a difficulty in particular in TEA-lasers having a rapid pulse sequence and gas circulation.
However, it is possible to omit an additional pre-ionization means if the side walls of the laser at the same time form the voltage supply means to an electrode. Disposed between the electrically conducting side walls and the second electrode is an insulation and thereon, on each side, is a glass plate [H. Jetter and K. Gurs, Optical Eng. 15, (1976), 17-19; G. J. Ernst and A. G. Boer, Optics Comm. 27, (1978), 105]. Due to the rapid rise in voltage at the beginning of the main discharge, a high field strength is very quickly produced between the upper electrode and the side walls. That produces a corona discharge for a period of from 10 to 30 nsec on the side walls which are covered with insulating material. When the corona discharge occurs, an UV-light pulse is produced, which provides the pre-ionization for the main discharge. The effectiveness of pre-ionization and thus the quality of the main discharge substantially depends in that arrangement on the geometry of the arrangement and the discharge voltage.
However, the last-mentioned arrangement is not suitable for TEA-lasers with a rapid pulse sequence as the laser gas which flows transversely with respect to the optical axis must be as far as possible exchanged twice, over the discharge volume, between two laser pulses, and therefore there is no possibility of using strip conductors, plates, walls, etc., which connect the two electrodes.